BUREAU OF SOILS. 209 



plant asli, while not yet sufficiently advanced to make final state- 

 ments advisable, appears to show that very many if not all of the 

 elements found in the soil usually find their several ways into the 

 Hving organism, and important economic as well as scientific data 

 are being accumulated for the further elaboration of the plant 

 physiologists. 



SOIL EROSION. 



There are but a few States where soil erosion is not a problem of 

 economic importance, and in some of the States, notably in certain 

 regions of the South and Middle West, it is sometimes of the first 

 importance to their agricultural development. Long study has 

 shown that there are two general methods of protection: (1) To 

 increase the capacity of the surface soil to absorb moisture, as by 

 deep and thorough tillage, and by the incorporation of organic 

 matter, thorough liming, etc.; and (2) by diminishing the rate of 

 flow of surface waters, as by terracing, contour ploughing, breaks 

 in gullies, etc. There appears to be reason for suspecting that in 

 certain cases, at least, the inherent properties of particular soils 

 seriously affect their liability to erosion, as in the case of certain 

 rather heavy soils containing a large percentage of mica, which 

 soils "flow" very readily when in contact with excessive quantities 

 of water. Hence, a laboratory investigation is in progress of soils 

 from different parts of the United States where erosion is a promi- 



nent factor in agriculture. 



SOIL MOISTURE. 



It has come to be recognized that every physical property of a 

 soil affecting the growth of crops is itself determined by the moisture 

 content of the soil and that there is a critical moisture content 

 characteristic of each particular soil at which the aggregate of physi- 

 cal properties produces an optimum condition for plant growth. 

 Consequently, the determination of this critical or optimum water 

 content has a practical importance and a comparatively quick and 

 rapid method for its determination has been developed. , 



SOIL PRESSURE. 



If a soil containing less than the critical moisture content be 

 further wetted, there are pressures developed which are of enormous 

 dimensions on the surface of the individual grains as determined by 

 theoretical calculations, and which are often of considerable magni- 

 tude on the surfaces of the soil mass. Pressures of 600 pounds or 

 more to the square inch of soil mass have been observed in the work 

 of the past year. These observations have a considerable interest 

 in theoretical and practical studies on tilth. But they have a much 

 more direct importance to engineers and practical constructors hav- 

 ing to deal with dams, earthworks, foundations, or other like prob- 

 lems where the earth is liable to continual or intermittent contact 

 with water. 



SOIL FERTILITY INVESTIGATIONS. 



The fundamental questions on which the fertility of amcultural 

 lands depends have been further elucidated by the research work of 

 the laboratory and field investigations of this office. The laboratory 



22814°— AGR 1915 14 



